Sandia's Ion Beam Laboratory looks at advanced materials for reactors

Technologist Daniel Buller stands in front of the beamline that connects the tandem accelerator to the transmission electron microscope (TEM) at Sandia's Ion Beam Laboratory. The blue light in the cylinder in the background above the computer screen is phosphor luminescing from proton beams hitting it. Photo: Randy Montoya

Sandia National
Laboratories is using its Ion Beam Laboratory (IBL) to study how to rapidly
evaluate the tougher advanced materials needed to build the next generation of
nuclear reactors and extend the lives of current reactors.

Reactor operators
need advanced cladding materials, which are the alloys that create the outer
layer of nuclear fuel rods to keep them separate from the cooling fluid. Better
alloys will be less likely to deteriorate from exposure to everything from
coolant fluids to radiation damage.

Operating a reactor
causes progressive microstructural changes in the alloys used in cladding, and
that can hurt the materials’ integrity. However, present-day methods of
evaluating materials can take decades.

The IBL, which replaced
an earlier facility dating from the 1970s, has been in operation for about a
year and is doing in situ ion irradiation experiments, potentially shaving
years off testing. The ion beams use various refractory elements to simulate
different types of damage and thus predict the lifetimes of advanced reactor
claddings. Some of the research was highlighted in a presentation by materials
scientist Khalid Hattar in December at the Materials Research Society
conference in Boston
in a paper co-authored by Tom Buchheit, Shreyas Rajasekhara, and B.G. Clark.

Researchers, trying
to understand the changes as a function of radiation dose, inserted a beamline
from the tandem accelerator, the IBL’s largest, into a transmission electron
microscope (TEM). This allows them to do in situ ion irradiation experiments at
the nanoscale and record results rapidly and in real time. Sandia’s laboratory
is one of only two facilities in the U.S. and one of only about 10 in
the world that can do this, Hattar said.

"The idea is to come
up with new ways to make different alloy compositions and different materials
for next-generation reactors and to understand the materials used in the
current-generation reactors," he said. "Then we can find ways of doing a
combination of TEM characterization as well as small-scale mechanical property
testing in this rapid testing scenario to screen these materials and see which
ones are the most suitable."

Better understanding
of cladding materials could help improve reactor efficiency.

To that aim, Hattar
and his team are using the IBL's capabilities to try to gain a fundamental
understanding of how the materials evolve in extreme environments at the
nanoscale. They hope that understanding can then be related to events on the
macroscale.

Take, for example,
something familiar like rust on a little red wagon.

"If you look at rust,
it's nonuniform," Hattar said. "So the location where that first rust starts to
occur must be related to some heterogeneous aspect of the microstructure. If we
can really understand on the nanoscale what causes it, that initiating factor,
then we can prevent the initiation, and without the initiation, you’ll never
have that rust formation."

The team developed a
system for testing cladding materials that Hattar believes can be used for
experiments under extreme conditions to simulate real-life environments.
Researchers can work with temperatures up to 1,200 C (2,192 F) and pressure up
to one atmosphere as well as ion irradiation to gain basic understanding of
radiation damage.

A recently completed
Laboratory Directed Research & Development (LDRD) program worked with a
variety of samples, everything from high-purity, single-crystal copper to
materials used in today's reactors. The team found that under the right
conditions, a combinatorial approach can be used with new alloy compositions
produced in-house, Hattar said.

The LDRD project
demonstrated a fundamental physics simulation of what's happening to the
material. In the next step, Hattar has suggested Idaho National Laboratory expose
selected materials to neutrons and then try them out in a real reactor. Since
the IBL can run experiments in as little as a day, researchers aim to pinpoint
the best material so the Idaho
laboratory, whose tests take much longer, won’t waste time testing poorer
materials, he said.

In one experiment,
the team examined both the composition of and effects of radiation on an alloy
being considered for the next generation of reactors, seeking the best
composition for different radiation exposures.

"Really understanding
how the microstructure evolves lets us know a lot about how the material will
perform," Hattar said. "So if we can rapidly determine how the microstructure
evolves and understand the mechanisms that it evolves by, we could gain a lot
of insight into what happens in the material."

That idea prompted
the test in which the team connected the large tandem accelerator to the TEM.
The tandem accelerator produces the radiation that is sent into the microscope,
allowing team members to watch as they irradiate materials. The TEM setup
achieves nanometer-scale resolution with results recorded in real time.

In the next generation
of testing, the team will have the option of using the beam from a Colutron
accelerator recently added to the IBL system.

"This will allow us
to do irradiation with both the heavy ions for displacement damage as well as a
helium-deuterium combination for understanding hydrogen embrittlement and
swelling of the cladding materials," Hattar said.

Once researchers
understand those mechanisms, they can go about selecting the proper material
for a wide variety of extreme environments where radiation plays a significant
role, such as shielding microelectronics from radiation from space.

The in situ ion irradiation TEM includes two
new stages that give the IBL even more unique capabilities. A microfluidic
stage allows researchers to look at fluids inside the microscope in real time,
and a vapor phase environmental cell can be used to study how corrosion occurs
as a function of time. The vapor phase studies were made possible by B.G. Clark
and Brad Boyce with Basic Energy Sciences funding. The microfluidic stage was
supported by Readiness in Technical Base and Facilities, an NNSA program.

Hattar said the team
is planning basic science applications to study fundamentals such as oxidation
mechanisms in materials, as well as gas phase flow experiments to look at the effects
of hydrogen or corrosive gases on samples. The system also can do tomography,
which enables the researchers to turn 2D TEM projection images into full 3D
reconstructions. This results in visualization of the sample from all angles
simply by compiling 2D images taken at a range of sample tilts.

IBL researchers hope
to provide fundamental insights into a variety of projects, both at Sandia and
externally, that are interested in everything from corrosion to radiation
damage to materials, Hattar said.

Experiments, however,
are in the early stages because the facility is so new.

"We're sitting in a
situation where we're just trying to see where they'll lead right now," Hattar
said. "It should be fun and there should be lots of different directions."